1. Field of the Invention
The present invention relates to an image forming apparatus using electrophotography or electrostatic recording, such as a copying machine, a printer, a facsimile machine, or a complex machine of these applications, and particularly to a structure for forming, on an image bearing member, a toner image used for adjustment.
2. Description of the Related Art
Conventionally, for example, in an electrophotographic image forming apparatus, an electrostatic image (latent image) formed on an electrophotographic photoreceptor as an image bearing member is developed with toner to form a toner image. After that, the toner image formed on the photoreceptor is eventually transferred and fixed onto a recording material (recording sheet, OHP sheet, or the like), and output to the outside of the image forming apparatus. As a system for transferring the toner image on the photoreceptor to the recording material, there is an intermediate transfer system in which the toner image formed on the photoreceptor is once transferred onto an intermediate transfer member and the toner image on this intermediate transfer member is transferred onto the recording material. Further, as the intermediate transfer system, there is known a tandem structure in which multiple image forming stations are lined up in the rotational direction of the intermediate transfer member to superimpose and transfer each color toner image formed in each image forming station sequentially onto the intermediate transfer member.
As described in Japanese Patent Application Laid-Open No. 2003-202711, there is known a density correction technique in which a toner image for adjustment (hereinafter called a patch) is formed on a photoreceptor or an intermediate transfer member to have a predetermined density, and the patch density is detected to perform feedback on imaging conditions according to the degree of the patch density. There is also known a technique in which relative position of patches for respective color components is detected to correct registration errors among respective color components.
In the meantime, as for the detection of the density and position of a patch (patch detection) as mentioned above, it is conceivable that, in an image forming apparatus for forming a multicolor image, all the multiple colors are formed together on an intermediate transfer member and read on the intermediate transfer member together. However, the intermediate transfer member varies in surface roughness and surface color with long-term use, and this may cause a substrate signal of the intermediate transfer member to fluctuate a lot. For example, the surface roughness is increased by the adherence of a paper loading material, or the surface is whitened by the adherence of external additives for toner. When the substrate signal of the intermediate transfer member is unstable, since the toner density is calculated as a contrast with the substrate upon patch detection, the accuracy of detection is reduced in the case of a color having low contrast with the substrate (e.g., black).
On the other hand, it is also considered that the patch detection is made on a photoreceptor, which does not come into direct contact with a recording material. In this case, since there is no substance transferred from the recording material, the surface durability fluctuation is small compared with that of the intermediate transfer member. This is preferred in terms of the stability of the substrate signal. Therefore, it can be considered that the detection of a black patch that is especially low in brightness and easily influenced by substrate fluctuations is made on the photoreceptor and the detection of patches for bright color components such as magenta, cyan, and yellow are made on the intermediate transfer member.
When such a structure is applied to a tandem image forming apparatus, it is preferred to place a black image forming station in the most downstream position in order to shorten FCOT (First Copy Out Time) upon formation of an image in a plain color of black.
However, in such a structure, when other color patches formed in the upstream stations pass through a primary transfer portion for transferring a toner image from a black photoreceptor in the downstream station onto an intermediate transfer member, the other color patches can be retransferred onto the photoreceptor. When retransfer occurs, the accuracy of detection of the other color patches on the intermediate transfer member may be reduced.
For example, as illustrated in FIG. 9, when an ordinary transfer bias is applied upon transfer of a toner image from a photosensitive drum 1 as the photoreceptor onto an intermediate transfer belt 51 as the intermediate transfer member in a primary transfer portion T1, a discharge phenomenon occurs downstream of the primary transfer portion T1. Here, if the toner charge polarity is negative, since negatively charged toner is electrostatically transferred onto the intermediate transfer belt 51, a positive charge is injected from a primary transfer roller 6 as a primary transfer member into the intermediate transfer belt 51. Then, the intermediate transfer belt 51 is charged with positive polarity to transfer the negatively charged toner onto the intermediate transfer belt 51. At this time, since a large potential difference occurs in a gap between the photosensitive drum 1 downstream of the primary transfer portion T1 and the intermediate transfer belt 51, a discharge phenomenon occurs in the gap downstream of the primary transfer portion T1 after toner transfer.
When such a discharge phenomenon occurs, since charge transfer occurs to reduce the potential difference between the intermediate transfer belt 51 and the photosensitive drum 1, the positive charge moves from the intermediate transfer belt 51 toward the photosensitive drum 1. In this case, since the positive charge is shot into toner held on the intermediate transfer belt 51, the toner may be reversed to the positive polarity. This causes the toner positively reversed to move in the direction of the photosensitive drum 1. The above is a retransfer generation mechanism. Thus, when the primary transfer bias is positive polarity as polarity opposite to the toner charge polarity, the bias can be set low in the positive direction to reduce the retransfer of toner.
On the other hand, it can be considered that the bias to be applied is of negative polarity that is the same polarity as the polarity of toner to pass the toner image on the photosensitive drum 1 through the primary transfer portion T1 without being transferred onto the intermediate transfer belt 51. Even in this case, however, the retransfer of toner can occur as follows. Namely, when a negative charge is supplied from the primary transfer roller 6 to the intermediate transfer belt 51, it electrostatically repulses the negatively charged toner supplied to the primary transfer portion T1. The toner acts to flick the toner electrostatically from the intermediate transfer belt 51 toward the photosensitive drum 1. Therefore, when the primary transfer bias is of negative polarity that is the same polarity as the toner charge polarity, the bias can be set low in the negative direction to reduce the retransfer of toner.
Thus, in order to prevent a patch on the intermediate transfer member from being retransferred at a downstream station, a bias to be applied to the primary transfer portion needs to be changed from the ordinary transfer bias. However, if the bias is switched from the ordinary transfer bias to a bias for preventing retransfer, the state of the surface potential of the image bearing member downstream of the primary transfer portion is changed. Then, when image formation is performed by using a portion where the state of the surface potential is changed, an image to be formed may be affected.
This point will be described below. As mentioned above, when an electric discharge occurs downstream of the primary transfer portion, the behavior of the positive charge to shoot into toner is seen, and the behavior of the positive charge to shoot into the surface of the image bearing member after passing through the primary transfer portion also occurs concurrently. In other words, as illustrated in FIG. 10, when an electric discharge occurs downstream of the primary transfer portion T1, the positive charge is absorbed to the surface of the photosensitive drum 1. Downstream, a charged bias of negative polarity is applied to a charging roller 2 as a charging member to absorb the negative charge from the charging roller 2 to the photosensitive drum 1 in order to charge the surface of the photosensitive drum 1 to a predetermined potential. Then, image formation is performed in the following processes: The surface of the photosensitive drum 1 charged to the predetermined potential (the potential of a non-exposed portion) is exposed by an exposure member, not illustrated, to form an electrostatic latent image, and the electrostatic latent image is developed with toner by a developing member, not illustrated.
Here, the photosensitive drum 1 has such characteristics that, when an inner photosensitive layer is exposed, a carrier is generated and the positive charge drifts up to near the drum surface layer, so that an exposed portion becomes at a potential more positive than the non-exposed portion, thereby forming the electrostatic latent image. However, the moving direction of the carrier is restricted by a carrier transport layer formed more on the surface layer side than a carrier generation layer, making it difficult to attenuate the carrier on a base layer side because the carrier passes through the carrier transport layer.
Thus, it is difficult to attenuate the positive charge shot into the surface of the photosensitive drum 1 due to an electric discharge occurring downstream of the primary transfer portion T1. Therefore, a negative charge is supplied by the charging roller 2 to cancel out the positive charge while charging the photosensitive drum 1 uniformly to prepare the surface of the photosensitive drum 1 for subsequent image formation.
However, when the bias to be applied to the primary transfer portion is switched from the ordinary transfer bias to prevent the retransfer of patches as mentioned above, a change is made in the amount of positive charge shot into the surface of the photosensitive drum 1 due to the electric discharge downstream of the primary transfer portion T1. Therefore, even if the surface of the photosensitive drum 1 to which the switched bias is applied is charged by the charging roller 2, the state of the surface potential of the photosensitive drum 1 will vary compared with a case where the ordinary transfer bias is applied. Then, when the surface of the photosensitive drum 1, the surface potential of which has varied, is used to perform the image formation processes such as exposure and development, an image to be formed will be displaced from a desired state.